The present invention involves methods and apparatus for rethreading tubular members which have been damaged or which were incorrectly threaded during the original machining operations. More particularly, the methods and apparatus are especially advantageous for use on numerically controlled (NC) machines and computer numerically controlled (CNC) machines for cutting threads on these members.
In the oilfield industry, there are various types of tubular members utilized in the drilling and production of underground hydrocarbon formations. Almost universally, these tubular members are made of a high-strength material, such as steel and the members are made in extended sections approximately 30 feet in length, joined together at each end by threaded connections.
Threaded tubular members for use in oil well drilling and production generally fall within a few narrowly defined classes. For example, tubular members for well drilling are called "drill pipe" and a number of sections of drill pipe connected together are collectively termed a "drill string". Usually, the threaded ends of drill pipe are called "tool joints" and these are manufactured separately from the pipe, then welded to the blank pipe to form drill pipe. Each section of drill pipe has an externally threaded (or male) end called a "pin", and an internally threaded (female) end called a "box". The most common type of thread design on tool joints is the tapered thread which allows for rapid connection and disconnection of the drill pipe sections, because it requires only a few turns to "make up" or "break out" a joint.
A second class of tubing used in the oil patch is the well casing which is larger diameter pipe with a thinner wall. Casing is used to line the well bore to prevent cave-in of the well bore, contamination of water strata, and loss of valuable hydrocarbon fluids through low pressure porous strata. Casing joints are usually termed "premium threaded" joints and may comprise tapered, cylindrical, or stepped cylindrical threads. Since casing is not removed or "tripped" from the well as often as drill pipe, it usually does not use the tapered connection, or it may have a very slight taper in the threads.
Tubing is the third category of oil well tubular pipe and is used to conduct hydrocarbons out of the wellbore or to insert mud, water, or treatment chemicals into the producing formation. Tubing is usually run inside of the casing of the wellbore, and may have tapered threads, cylindrical threads, or stepped threads.
All of these types of threads are well known in the industry. For example, the highly tapered drill pipe thread is disclosed in the patents to F. Stone, U.S. Pat. No. 1,932,427 (issued 10/31/33); S. Evans, U.S. Pat. No. 2,772,899 (issued 12/4/56); and E. Wehring et al, U.S. Pat. No. 3,047,316 (issued 7/31/62). Cylindrical threads are disclosed in the patents to C. L. Griffin, U.S. Pat. No. 2,636,753 (issued 4/28/53), and G. S. Knox, U.S. Pat. No. 2,907,589 (issued 10/6/59). Stepped cylindrical threads on oilfield tubulars are disclosed in the two patents to M. D. MacArthur, U.S. Pat. No. 2,992,019 (issued 7/11/61) and U.S. Pat. No. 3,100,656 (issued 8/13/63). The forming of the threaded portions of the tool joints and Premium Threaded Ends is very critical because of the strict requirements of high strength; and in some cases, high-pressure fluid tight sealing characteristics in the threaded joints.
In most modern threading operations, the cutting of the threaded sections on the tool joints and premium threaded ends is accomplished by computer controlled equipment, some of which are commonly called NC machines and CNC machines. These two types of automated machines utilize numerically actuated tape controls to provide automated machining functions on the tubular sections being threaded. The automated controls and automated machinery have been successful in speeding production of threaded members manyfold over the older conventional types of lathes and milling machines which were commonly used prior to the adoption of NC and CNC systems. One area in which the NC and CNC machining systems has not improved, is in the reworking of incorrectly threaded sections and damaged threaded sections of the tubular members. The methods currently in use for reworking of threaded sections are those that have been used for many years and are very slow and time-consuming. In fact, the conventional reworking techniques most often utilize more labor than can be recovered in the value of the part; and consequently, most threaded parts which are incorrectly threaded or which have damaged thread sections, are scrapped rather than reworked.
For example, a production machining operation using an NC machine, can produce up to 18 tool joints per hour in its threading operation. The NC machine is almost totally automatic and requires only the worker to set up the blank piece in the chuck or collet of the machine. Then the proper program is activated at the control panel and the indexing head of the machine performs all of the necessary threading and facing operations. The speed of the NC machine allows the high production rate of up to 18 threaded members per hour.
The conventional reworking technique utilizing the available technology is a three-step process. First, the damaged or improperly threaded piece is set up in a turret lathe to reface and rebore the seal face on the end of the joint. In the second step, the threads are milled on a thread mill to obtain a new thread profile. Then, in the third step, the face is "gaged" by removing a thin layer of material to obtain the exact "standoff" or spacing between the end of the joint and the threads themselves. These three reworking techniques generally require from 30 minutes to an hour on each piece, resulting in a production of reworked pieces of only one or two per hour. This is approximately 10 to 20 times slower than the normal NC production rate, and thus is economically unfeasible. Thus, the economics led to scrapping of the damaged or incorrectly threaded section, rather than attempting to rework the section and correct the defects or damaged portions.
Other methods of reworking damaged and defective threaded tubular goods utilizing NC and CNC machines are likewise as complicated and time-consuming as the older conventional machining techniques utilizing the turret lathe and the thread mill. The conventional techniques for reworking using the computer-assisted machines involves setting up a number of dial indicators and measuring instruments upon each piece to be reworked to ensure that the threaded member is located exactly in the correct position, both rotationally and axially with respect to the spindle and turret of the NC machine. It is critical that the piece to be reworked is clamped in the collet section of the lathe in the exact orientation with respect to the rotational axis as well as the longitudinal axis, which is necessary because the reworking is done by removing exactly one or more threads from the threaded section and cutting a new thread into the existing shoulder of the old thread. If alignment is not exact with respect to rotational and axial directions, it is possible that the new thread profile would extend into the already open area cut between the old existing threads. This would result in a second defective thread, having an improper thread profile.
Thus, the present invention provides an improvement over the conventional reworking techniques, which utilize the turret lathe and thread mill, as well as significant improvements over the conventional reworking techniques utilizing NC machines with complicated measuring instruments for locating the defective piece in the NC collet. The present invention achieves this economically and in a relatively short period of time by utilizing a special alignment technique, taking advantage of a master threaded piece and a threaded adapter having locator means formed in the adapter body.